In recent years, studies on gas sensors capable of detecting various gases such as CO2 and NH3 have been conducted, and among them, gas sensors using carbon nanotubes (CNTs), for example, have been especially attracting attention from the viewpoint of detection sensitivity for gases, miniaturization, and energy saving (for example, see Patent Literature 1 and Non Patent Literature 1). In fact, the gas sensor using a carbon nanotube like this has a configuration in which the surfaces of the carbon nanotube provided between a source electrode and a drain electrode are chemically modified with two kinds of polymers, in order to detect CO2 that is a target of detection, for example. Further, the gas sensor is configured in such a manner that the carbon nanotube is disposed on the silicon back gate via a silicon oxide film, and a gate voltage can be applied to the silicon back gate.